The devices involving heat exchanges over gases and liquids at low temperatures (<0° C.) generally require brazed aluminum exchangers in order to benefit from their compactness, lightness and low cost with regard to other solutions that withstand low temperatures (stainless steel in particular).
FR-A-2947329 describes a process for cooling a flow containing at least 35% carbon dioxide and mercury in a first heat exchanger made of stainless steel to a temperature of −30° C. and from −30° C. in a second heat exchanger made of aluminum. The first exchanger is designed to allow the accumulation of mercury and after the first exchanger a purification step makes it possible to reduce or even eliminate the mercury upstream of the second exchanger.
The use of a brazed aluminum heat exchanger following a stainless steel exchanger may prove problematic due to the residual mercury that will solidify therein. The solidification in an aluminum heat exchanger may lead to plugging and ultimately to embrittlement of the aluminum during shutdown phases.
The presence of mercury poses a problem since in the dry phase the liquid mercury amalgamates with aluminum and pierces the exchangers (the well-documented phenomenon of liquid metal embrittlement or LME). This phenomenon is above all linked to the deposits of mercury as indicated in “Risk analysis for operation of aluminum heat exchangers contaminated by mercury” by Wilhelm, Process Safety Progress, vol. 28, no. 3, 2009. It is well known to stop the mercury with the aid of sulfur-doped activated carbon beds, especially in the natural gas liquefaction industry.
This solution is nevertheless expensive and requires frequently changing the adsorbent. The invention presented here aims, inter alia, to reduce the impact of the mercury on the cost of the apparatus.
Furthermore, it is known that certain impurities prohibit the use of such and such an adsorbent or greatly reduce the effectiveness thereof. Thus, the presence of NOx, even in small amounts (a few ppm), makes the use of activated carbon very tricky as NO2 can be absorbed thereon and, since it is a powerful oxidizing agent, can initiate the combustion of the activated carbon after accumulation, over time, of significant amounts of NO2. It should be noted that if the gas contains NO and oxygen, the oxidation reaction of NO may be catalyzed by the adsorbent itself.
It should be noted that other adsorbents that aim to stop the mercury have the property of being non-combustible (for example sulfur deposited on an alumina support), nevertheless they all have a reduced effectiveness for stopping the mercury due to the presence of NO2 which is co-adsorbed and blocks the active sites.